Lay summary
The Fribourg Atomic Physics group (FRAP) has a world-leading expertise in the study of atomic and molecular defects in solid helium matrices. This unique variant of matrix isolation spectroscopy in quantum solids is closely related to defect spectroscopy in heavier rare gas and hydrogen solids, in doped helium nano-droplets, and in superfluid helium.

On one hand, we are interested in gaining a deeper understanding of the modification of free atomic properties by cubic and hexagonal solid helium matrices, which, together with model calculations, gives us a quantitative insight into the structure, symmetry, and dynamics of the local trapping sites in that unusual matrix. On the other hand, the unique properties of the quantum solid 4He allows the formation of structures which cannot be observed under other conditions, such as the formation of large exciplex molecules or the macroscopic iceberg structure which we have recently discovered. According to our hypothesis, the latter contains positively and negatively charged particles - ionic snowballs and electron bubbles - bound together by the Coulomb interaction. We study both types of dopants using spectroscopic methods in infrared, visible, and near ultraviolet domains. We also apply optical and magnetic resonance spectroscopy to embedded neutral alkali atoms to obtain information on the crystalline structure and density of the iceberg. We also study metal-helium exciplexes and search for new complexes formed by highly excited states of Cs and ground-state He atoms.